In order to meet ever increasing consumer and governmental requirements for fuel economy, performance, and emissions, the trend in engine fuel systems is to change from a centrally located fuel pump with multiple plungers connected via fuel pathways to each combustion cylinder (commonly known as pump and line fuel systems), to unit fuel pumps/injectors which are placed over the combustion cylinder. In this way, the volume associated with fuel pathways is greatly reduced enabling higher injection pressures to be obtained and eliminating fuel line dynamics, leading to better engine performance.
Although not shown, in general, unit pumps differ from unit injectors only by the fact that the injector body is not placed over the combustion cylinder. Instead, the main body of the device is placed remotely from the combustion cylinder and connected by means of a heavy-duty fuel line to the injection tip, which is over the combustion cylinder. Remote positioning of the main body of the device allows more space in the cylinder head to be allocated for intake and exhaust valves. It also makes possible a more stiff drive mechanism, especially if the push rod/rocker arm mechanism is eliminated. Both mechanical and electronic unit pumps are possible.
Even pumps/injectors (hereinafter referred to as "injectors") have certain undesirable characteristics. Namely, it is difficult to achieve low minimum controllable fuel delivery volume. This is a problem because when the amount of fuel required by an unloaded engine is less than the minimum controllable fuel delivery volume of the injector, misfire or instability can occur. Also, the rapid release of hydraulic pressure in the injectors after injection causes high loads, noise, and wear in the plunger and plunger drive mechanism.
FIG. 1 of the drawings shows a mechanical injector and its drive mechanism. Working of the drive mechanism and injector is explained later under the "BEST MODE" section of this case. However, for present purposes, it is important to understand the failings of such an injector. When fuel is being ejected, significant energy is stored in the drive mechanism due to deflection of its components and supports. When injection is finished, pressure on the plunger drops rapidly. This releases the load on the drive mechanism and allows it to spring apart or separate. The separation usually occurs at the push rod ends, but may also occur at other interfaces. A short time after separation, those parts of the drive mechanism which have separated are pushed back into contact by the injector return spring. The resulting impact causes high loads, noise, and wear of components in the drive mechanism and the gear train which drives the cam shaft.
Referring now to FIG. 2, an electronic injector is shown. The main difference between mechanical and electronic injectors is the method in which the fuel is bypassed from the fuel chamber to end or control injection. Although an electronic injector has a plunger, it is much simplified when compared to the mechanical injector. This is because there is no need for a rack bar, nor for the gear to rotate the plunger, nor for the scroll to be cut into the plunger for the purpose of starting and stopping bypass flow (these terms and their purpose are explained fully in the "BEST MODE" section of this case).
Bypass of fuel from the fuel chamber is controlled by a solenoid. With the solenoid de-energized, fuel can escape from the fuel chamber through a pathway, past the poppet seal land, and exit by way of another pathway to the fuel supply manifold. Therefore, no appreciable pressure is maintained below the plunger and the drive mechanism components can spring apart.
When the solenoid is energized, the poppet is pulled upward causing the poppet seal land to seat and shut off bypass flow. The pressure then increases in the fuel chamber and injection occurs in a manner quite similar to the mechanical injector. Injection ends when the solenoid is de-energized and the bypass is re-opened.
Now looking at FIG. 3, one attempt for improving an injector's minimum controllable fuel delivery volume is known as spill pulse assisted needle closure (SPANC), which is intended to provide a more rapid closure of the needle valve. It is also intended to reduce high loads, noise, and component wear in the drive mechanism by maintaining sufficient pressure in the fuel chamber until the cam follower reaches maximum lift. In an injector with the SPANC device, when the plunger nears the end of its pumping stroke, the bypass port is uncovered. The pressure pulse of fuel from the fuel chamber out the bypass port is restricted by an orifice and a portion of the pulse is re-directed through a pathway to a piston which sits above the needle valve. The pressure pulse force on the upper surface of the piston causes it to quickly move down against the needle valve, thereby quickly closing the needle valve.
Contrary to the original intent, tests of a SPANC device have shown that the minimum controllable fuel delivery volume is actually increased. This apparently follows from the fact that because the pressure pulse applied to the top of the piston also makes its way to the needle valve fuel chamber, flow out the tip orifices is at a faster rate, thus an increased volume of fuel is delivered before the pressure in the needle valve fuel chamber dissipates enough for the needle valve to close.
The present invention is intended to solve problems inherent in prior injectors by improving minimum controllable fuel delivery volume and by maintaining enough pressure in the fuel chamber at the end of injection to eliminate the ability of the drive mechanism components to separate.